When drilling for oil and gas, it is desirable to steer the drill bit in a chosen direction, notwithstanding that the drill bit and other components of the downhole steering assembly might be several kilometers below the Earth's surface. Downhole steering assemblies, which can achieve the desired directional drilling, are in widespread use, and can often drill complex borehole paths in which the trajectory of the drill bit is controlled during the drilling operation.
Directional drilling is complicated by the necessity to operate the downhole steering assembly within harsh borehole conditions. The steering apparatus is typically disposed near the drill bit. In order to obtain the desired real-time directional control, it is preferred to operate the steering apparatus remotely from the surface of the Earth. Furthermore, the steering apparatus must be operated to maintain the desired path and direction regardless of its depth within the borehole and whilst maintaining practical drilling speeds. Finally, the steering apparatus must reliably operate under exceptional heat, pressure and vibration conditions that will typically be encountered during the drilling operation.
Directional drilling applications require the drill string, or parts of the downhole steering assembly, to articulate and/or be flexible so as to pass along the curved borehole. Many prior art documents disclose components suitable for use in directional drilling. U.S. Pat. No. 4,904,228 in particular describes a universal ball joint permitting articulation of respective parts of a downhole assembly. U.S. Pat. No. 5,503,236 describes another universal joint mounted adjacent to the drill bit. U.S. Pat. No. 4,880,067 describes a downhole assembly including a number of articulating sections, the articulating sections carrying a flexible drive shaft which connects the output shaft of a downhole motor to the drill bit.
U.S. Pat. No. 2,740,651 describes a drill bit which is mounted by way of a resilient flexible coupling. U.S. Pat. No. 3,135,103 describes a flexible joint for a drill string. U.S. Pat. No. 3,446,297 describes a flexible drill collar. U.S. Pat. Nos. 2,402,003 and 3,578,029 disclose flexible pipes suitable for use in drill strings.
U.S. patent application 2011/0308858 discloses a flexible joint which can be used in a downhole steering assembly. The flexible joint has a constant velocity coupling and a spring which acts upon the coupling. The spring can be used to adjust the force which is required to bend the constant velocity joint, and also to adjust the maximum angle through which the joint can be articulated.
Many types of steering apparatus are known. A common type of steering apparatus comprises a downhole motor disposed in a housing with a longitudinal axis, at least part of which axis is offset or displaced from the axis of the borehole. The motor can be of a variety of types including electric and hydraulic. Hydraulic motors which operate by way of the circulating drilling fluid are commonly known as a “mud” motors. The drill bit is attached to the output shaft of the motor, and is rotated by the action of the motor. The axially offset motor housing, commonly referred to as a bent subsection or “bent sub”, provides axial displacement that can be used to change the trajectory of the drill bit. By rotating the drill bit with the motor and simultaneously rotating the motor housing with the drill string, the orientation of the housing offset continuously changes and the path of the advancing borehole is maintained substantially parallel to the axis of the drill string. By rotating the drill bit with the motor only, the path of the borehole is deviated from the axis of the non-rotating drill string in the direction of the offset of the bent sub. By alternating these two methodologies of drill bit rotation, the path of the borehole can be controlled. A more detailed description of directional drilling using the bent sub concept is disclosed in U.S. Pat. Nos. 3,260,318, and 3,841,420.
It is a recognised disadvantage of a downhole steering assembly using a bent-sub that the drill bit is required to drill an over-large hole whilst drilling a linear section of borehole. It is another disadvantage that the drill string must not rotate when a curved section of borehole is being drilled, which increases the frictional resistance to the advance of the drill bit along the borehole. Furthermore, the degree of curvature is fixed by the bend of the housing, so that the operator must choose the optimum bend angle dependent upon the expected drilling conditions and the borehole curvature required.
UK patent applications 2 435 060 and 2 440 024 describe alternative methods of steering a drill bit by way of the bent housing of a downhole motor. These documents address the second disadvantage stated above by allowing the drill string to rotate continuously. There is a rotatable connection between the drill string and the bent sub. A clutch mechanism is provided within the rotatable connection, the clutch mechanism controlling the orientation of the bent sub and consequently the orientation of the bend.
Another type of steering apparatus comprises a steering component such as that described in our published European patent 1 024 245. This steering component allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth. Downhole steering assemblies incorporating steering components such as that of EP 1 024 245 avoid the disadvantages of the bent-sub assemblies described above.
However, steering components such as those of EP 1 024 245 are necessarily mechanically complex. It is a known concern for some operators that mechanically complex steering components such as those of EP 1 024 245 can be damaged by vibrations and the like. It will be appreciated that the forces acting upon a downhole steering assembly are considerable, and whilst the steering component can be manufactured to withstand the expected forces, unexpected and excessive forces can be experienced during uncontrolled vibrations within the downhole steering assembly.
It is necessary for the downhole steering assembly to be sufficiently stiff or rigid to communicate the torque to drive the drill bit, and also to communicate the lateral steering forces from the steering component to the drill bit. However, this inherent rigidity increases the likelihood that vibrations occurring at the drill bit will be communicated to the steering component. If the vibrations are close to the resonant frequency of a part of the downhole assembly the vibrations which are communicated to the steering assembly can be large and damaging.
The operator at the surface will typically monitor the status of the downhole steering tool continuously during use, and will seek to identify unwanted vibrations. The steering assembly can for this purpose carry accelerometers for example adapted to detect unwanted vibrations, the output of the accelerometers being communicated to the surface. The vibrations can be generated adjacent to the drill bit, or they can be generated within the downhole motor or drill string. The steering assembly can therefore experience unwanted vibrations reaching it from downhole or uphole, or both. If unwanted vibrations occur, the operator will typically seek to eliminate the vibrations, or at least disrupt their development, by reducing the weight on bit or by adjusting the rate of rotation of the drill string and/or the drill bit, or by all of these. The onset of damaging vibrations can be very rapid, however, and the vibrations can damage a steering component before they are eliminated.